The science of shark control (and what it means for the Western Australia cull)

OLYMPUS DIGITAL CAMERAProf Colin Simpfendorfer is the Director of the Centre for Sustainable Tropical Fisheries and Aquaculture at James Cook University. He has more than 25 years of experience in researching sharks, and has published extensively in the scientific literature on shark biology, ecology, fisheries and conservation. He is a graduate of James Cook University where he undertook both his undergraduate and postgraduate training. After completing his PhD he worked on shark fisheries at the Western Australian Fisheries Department before moving to Florida to work at the Center for Shark Research at Mote Marine Laboratory. He returned to JCU in 2007 to lead the Fishing and Fisheries Research Unit, where he has helped build a research group focused on improving our understanding of sharks and how best to conserve and manage their populations.

Call it a shark cull, shark control or bather protection, for decades governments have been trying to reduce the risk of humans being killed by sharks – by killing sharks. New South Wales, Queensland, KwaZulu-Natal (South Africa), Hawaii, Dunedin (New Zealand), Hong Kong, Somalia (during the US military intervention) and now Western Australia have, or had, shark control programs to reduce the risk of human-shark interactions.

Western Australia’s new program has sparked huge controversy, with many calling for the government to stop and pursue alternatives.There have been a range of claims that there is no science to support shark control. Many of these have been based on the effects of removing large predatory sharks on ocean ecosystems or that there is no evidence that shark culls reduce the risk of attack.Both of these are valid scientific considerations and need to be taken into account. However, neither addresses whether there is some scientific basis to shark control programs.

So here I would like consider whether there is a scientific basis to shark control programs. To do this I’ll look first at the theory, and then if there is evidence to support it based on analysis of data from the programs in KwaZulu-Natal and Queensland.

The theory

Shark control programs are based on the theory that by fishing close to popular swimming beaches local populations (not the entire population) of dangerous sharks are reduced, hence reducing the chance of human-shark interactions. This obviously takes some time to occur, and depends a lot on the movement patterns of sharks.

The evidence

Evidence from most shark control programs shows that over the first several months to years of fishing that there are large declines in the rate at which sharks are caught. What this means is that the control program is removing sharks faster than they move in from surrounding areas.

The most important question is whether there is evidence for localised declines in shark populations. The answer is yes. The best data comes from the KwaZulu-Natal Sharks Board. Here they have introduced new nets (to catch sharks, not exclude them from beaches) sometimes only a few kilometres from existing installations. When this happens catch rates at the new installation are very high, but fall off quickly and stay low indicating that there was a localised population that had not been particularly affected by the other installations. The best data for this comes from bull sharks, one of the main targets of the KwaZulu-Natal program. So there is evidence to support the theory on which shark control is based – that fishing will cause local depletion and hence reduce the risk of human-shark interactions.

The second question is whether there is evidence that this works for all species of sharks. The answer to this is definitely no. The best example of this is tiger sharks. Data from both KwaZulu Natal and Queensland has has demonstrated that catch rates of these species did not decline substantially (and even at times increased) over decades. Why? Most likely because tiger sharks are highly mobile and do not remain in one location for very long. This means that there is no local population to deplete and so catch rates are not reduced and thus neither is risk.  This also explains why the Hawaiian program, which was targeted at tiger sharks, was unsuccessful at reducing human-shark interactionsSo the effectiveness of shark control varies dramatically depending on the species.

Implications for Western Australia

I’ve already shown that the effects of shark control measures for two of the target species (bull and tiger sharks) of the Western Australian program may be different. Tiger sharks are likely to be unaffected (but they have not been identified as the species responsible for most sharks bites on humans in southern Western Australia) while bull sharks will be (although in my experience they are rarely caught in southern Western Australia).

The big question is really whether localized populations of white sharks will be reduced making Western Australian waters safer. Movement data on white sharks from Australia (and there is quite a bit of it) shows that white sharks, like tiger sharks, are wide-ranging and normally only stay in local areas for short periods of time (some of their feeding locations at permanent seal and sealion colonies are exceptions to this). Based on this, the available evidence suggests the Western Australian program is unlikely to achieve its aim of reducing white shark numbers close to popular beaches and hence reducing risk for ocean users. Evidence from the KwaZulu-Natal Sharks Board showing that catch rates of white sharks did not decline between 1978 and 2003 further supports the conclusion that the Western Australian program will not be effective at achieving its aim of keeping the public safe.